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The present invention relates to computed tomography (CT) imaging, and more particularly, to cardiac CT imaging.
In one type of a computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the xe2x80x9cimaging plane.xe2x80x9d The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of attenuation of the x-ray beam. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a xe2x80x9cviewxe2x80x9d and a xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a two-dimensional scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from two-dimensional data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
It is desirable to minimize the amount of time required to generate each image slice to minimize motion related image degradation. To reduce the total scan time, a xe2x80x9chelicalxe2x80x9d scan may be performed in which the patient is moved while the data for the prescribed number of slices is acquired. Such a process generates a single helix from one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
An alternative xe2x80x9chalf scanxe2x80x9d technique also has been developed in which x-ray attenuation data for each slice is acquired during approximately half a rotation of the gantry. The half scan technique requires half the time to acquire data for a slice image, as compared to a full rotation scan.
Computed tomography is frequently employed to image a patient""s heart. However, because the gantry requires time (e.g. one-half second) to make a full rotation, the continuous movement of the heart and the blood blurs the resultant images. Even the faster half scan technique still suffers from motion artifacts.
U.S. Pat. No. 6,275,560 describes an improved technique in which the x-ray emission from the CT system is gated by sensing the cardiac cycle of the patient and keying image acquisition to intervals during the cardiac cycle when the heart is relatively still. This technique uses an electrocardiograph (EKG) which produces an electrical signal representing the patient""s cardiac waveform. The QRS wave of that waveform is used to determine when diastole of the heart occurs and the x-ray emission is activated during diastole. To reduce motion artifacts further, the x-ray attenuation data for a first half of the views during scan are acquired during the diastole of one heart cycle and the x-ray attenuation data for the other half of the views are acquired during diastole of the next heart cycle. This technique, referred to the xe2x80x9cSnap-Shot Burst,xe2x80x9d in effect halves the time that the data is acquired during each cardiac cycle and thus less motion of the heart occurs during each acquisition period.
However, the cardiac gating techniques require knowledge of the cardiac activity in order that movement of the gantry and the table on which the patient is positioned can be synchronized with the acquisition intervals. The speed of the gantry has to be set at the beginning of the scan so that the emitter and detector will be in the proper angular positions during diastole. The table also must move at a speed in which the helical scan data is accurately acquired. Cardiac patients often suffer from arrhythmias, other heart conditions and anxiety which cause their heart rate to change unpredictably, thereby making selection of the gantry and table speeds difficult for an operator to determine.
A medical imaging system acquires image data at a plurality of views around a patient in an imaging plane. A method for operating such an imaging system comprises maintaining a database that contains a plurality of data items from a number of human beings. Regression analysis is performed on the plurality of data items in the database to derive an algorithm which defines an predicted heart rate value as a function of the plurality of data items. The algorithm then may be employed prior to imaging to predict a patient""s heart rate which is likely to occur during an imaging procedure.
To make that prediction, values for the plurality of data items are obtained for the patient. The algorithm is applied to the obtained data items to determine an predicted heart rate value for the patient. Operating parameters of the medical imaging system then are set in response to the predicted heart rate value for the patient.
In the preferred embodiment of the invention the data items relate to characteristics of the patient and the imaging system operation which affect the patient""s heart rate during an imaging procedure. For example, the plurality of data items are selected from a group comprising heart rate prior to imaging, heart rate during a previous imaging procedure, delay period between injection of a bolus and the bolus reaching a patient""s organ of interest, the type of the bolus, rate of injection of the bolus, length of time of the imaging procedure, the patient""s gender, patient""s weight, patient""s age and patient""s ethnic background.